天然气发动机曲轴箱爆炸模拟及分析

为了验证船用天然气发动机曲轴箱的设计强度能否承受最恶劣爆炸工况下的超压,利用CFD计算软件对曲轴箱内燃料-空气混合气体爆炸后果进行了数值模拟。现有的CFD计算方法普遍采用等效气体云模型,其中最新的Q9模型能够综合考虑气体膨胀率和层流燃烧速率对爆炸后果的影响,应用广泛。然而在曲轴箱等高拥塞度受限空间内发生的气体爆炸,火焰传播猛烈且燃烧状态复杂,对于这种情况采用Q9模型来进行模拟会造成结果的不准确,为此基于荷兰应用科学院(TNO)多能法在现有模型的基础上推导了适合曲轴箱内爆炸模拟的等效气体云模型。通过与试验数据的验证对比发现,新模型在高拥塞度较小容积受限空间中的计算结果精度较高,误差在20%以内,且混合气体越接近理想状态(化学计量浓度),该模型的计算精度越高。以某典型船用天然气发动机曲轴箱为例,采用新的等效气体云模型计算了最恶劣爆炸工况下曲轴箱内的超压分布,并导入有限元软件进行了强度评估。评估结果表明：曲轴箱内最大应力的位置发生在结构强度较弱的油底壳处,应力峰值为361.257MPa,油底壳采用Q235材料,该应力已超过其许用应力,该部分结构无法承受最恶劣爆炸工况下的超压,因此如不安装防爆阀,需在设计时对曲轴箱油底壳结构进行适当加强。

Simulation of Explosion Consequences on Marine Natural Gas Engine Crankcase

In order to demonstrate whether the design strength of natural gas engine crankcase can withstand overpressure in the explosion of the severest working condition, a numerical simulation of the explosionresults was carried out by CFD software. As a minimum approach, the equivalent stoichiometric fuel-air cloud method is widely employed for the purpose of performing explosion simulations to get a representative distribution of explosion load. The Q9 cloud model is the kinds of representative equivalent fuel-air cloud, in which the laminar burning velocity and volume expansion rate are taken into consideration. However, the fuel-air flow field turns into turbulent regime and the flame propagation is also in violent and turbulent status in engine crankcase. To give a more reasonable equivalent fuel-air size, the turbulent burning velocity must be consideration. A new equivalent approach is put forward based on the deduction from the TNO multi-energy method. By comparing to the test data, the new model can obtain a higher accuracy result in small volume space with high congestion, and the result error is less than 20%. The closer the ER value of the mixed gas is to the ideal state, the higher accuracy of the calculation model can achieve. To take a typical marine natural gas engine crankcase as an example, this paper adopts new CFD model to calculate the severest explosion condition in the crankcase and introduces the overpressure result to ANSYS to carry out strength evaluation. The result indicate: the position of the maximum stress in the crankcase occurs at the bottom of oil sump, and the peak stress is 361.257MPa. The material used for oil sump is Q235 steel which its permissible stress is less than the existing peak stress. Therefore, if anti-explosion valves are not installed, appropriate strengthen for the oil sump structure should be carried out in the design stage.